Process for the production of alkaline cyanates in sulfoxide and sulfone solvents



3,167,387 PROCESS FOR THE PRODUCTION OF ALKALNE CYANATES 1N SULFOXHDE AND SULFONE SOLVENTS William E. Erner, Brandywine Hundred, Del., assigner to Air Products and Chemicals, Inc., Philadelphia, Pa., a corporation of Delaware Filed Aug. 29, 1963, Ser. No. 305,333 4 Ciaims. (Cl. 23-75) The present invention is directed to the synthesis of alkali metal and alkaline earth metal cyanates from carbonates and urea by a low temperature process which avoids the conventional high temperature fusion technique.

Metal cyanates are prepared in current practice by a number of processes. In one method oxidizing agents such as oxygen, air, peroxides, PbOg, etc., convert metal cyanides to the cyanates. In another method, metal cyanides are converted to cyanates by electrolysis in caustic solution. However, because cyanates undergo hydrolysis in hot aqueous solution, the isolation of pure cyanates is difficult. One of the widely practiced commercial syntheses is carried out by fusion of metal carbonates and urea at about 550 to 600 C. Reference is made to US. Patent No. 1,915,425 for a description of a method involving such fusion of alkali carbonate and urea. However, it is diicult to obtain carbonate-free melts of metal cyanates even by this process. Furthermore, under high temperature fusion conditions, the principal cyanate product changes to carbonate or cyanide and ammonia. Samples of commercial potassium and sodium cyanate made by the fusion process have a carbonate content varying from a trace quantity to a predominance of carbonate. Moreover, the cost of producing alkali cyanates by the' fusion process has been great enough that there has been a continuing demand for an inexpensive method of making alkali cyanates.

1n contrast to this high temperature fusion process, the

process of the present invention provides a simple and l effective means for the production of alkali metal and alkaline earth metal cyanates at substantially lower costs.

'An object of the present invention is the low temperature synthesis of alkali and alkaline earth metal cyanates from the corresponding carbonates and urea wherein the reactants are in solution and the reaction is effected without fusion of the reactants.

A further object of the present invention is the synthesis of alkali and alkaline earth metal cyanates in the presence of an inert liquid which acts as a mutual solvent and heat transfer agent.

Y et a further object of the invention is the preparation of alkali metal cyanates in an economical manner not heretofore realized.

' f These and other objectives, as are discernible from the following specification, are accomplished in the following manner:

ln accordance with the present invention, alkali metal and alkaline earth metal cyanates are synthesized by dissolving the corresponding metal carbonates and urea in a non-aqueous solvent having a high solvency for the carbonate and urea, not dissociated under the conditions of the reaction and inert to the reactants, heating the solution to a temperature above about 80 C. and generally no higher than about 250 C. to accelerate the reaction and to remove volatile byproducts formed in the reaction, and then cooling to approximately room temperature or below to effect the precipitation of substantially pure metal cyanate from the solvent for said synthesis.

kMore specifically, the alkali metal or alkaline earth metal carbonate is reacted at temperatures in the range United States Patent O "ice of to about 250 C. with urea in an inert mutual solvent especially selected as having a high solvency for the metal carbonate and urea at reaction temperature but having very slight solvency for the alkali metal cyanate at ordinary temperatures. An exceptionally satisfactory solvent medium for such a process is dimethyl sulfoxide which boils at 189 C. and has unusually high solvency for alkali and alkaline earth metal carbonates, is stable under the conditions of reaction and inert to both the reactants and the reaction product. The reaction may be further promoted by sparging an inert gas such as nitrogen through the reaction mixture to facilitate the removal of water vapor and other volatiles formed in the reaction.

The process of the invention is particularly unique in effecting the synthesis of alkali metal and alkaline earth metal cyanates at temperatures significantly lower than the temperature of fusion of the corresponding metal cyanate or of a reaction product consisting principally of metal cyanate.

In a typical operation in accordance with the invention, as in the synthesis of sodium cyanate, one molar equivalent of sodium carbonate and at least two molar equivalents of urea are dissolved in dimethyl sulfoxide in a ratio of from about 3 to about l0 parts by weight of dimethyl sulfoxide per part of total reactants, with warming and stirring. As the reaction proceeds, ammonia, carbon dioxide and water vapor are released. These are removed overhead preferably in a stream of inert gas, such as nitrogen. The temperature is raised to approximately the boiling point of the solvent, i.e., 189 C. for dimethyl sulfoxide, and held for about 15 minutes until ammonia evolution ceases. The solution is then cooled and sodium cyanate precipitated. The resultant slurry is filtered or centrifuged and the product crystals washed with a volatile liquid (e.g., low boiling organic halide, hydrocarbon, or absolute alcohol) and then dried. The product is generally recovered in at least of theoretical yield and of high purity, as evidenced by a negative test for carbonate.

The synthesis is visualized by the following equation of the reaction:

wherein, if M is an alkali metal, n has a value of 2, as in lithium, sodium, potassium, rubidium or caesium carbonate, or wherein, if M is an alkaline earth metal, n has a value of l, as in beryllium, magnesium, strontium and calcium, whose carbonate salts are appreciably soluble in a stable solvent, which is preferably dimethyl sulfoxide, but may conceivably be a similar, stable, inert, relatively volatile liquid such as methyl ethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, dimethyl sulfone, methyl ethyl sulfone, diethyl sulfone and thiophane sulfone boiling in the range of from 80 C. to about 250 C. having an appreciable solvency for metal carbonates and urea at temperatures preferably in the range of C. to about 200 C. and slight solvency for cyanates at ordinary temperatures and unreactive in the system. All the named solvents correspond to the formula C.' II,.SOz in which the C4 Hy is provided by possibilities such as tetrarnethylene -CH2CH=CHCH2-dimethyl, diethyl, and methyl ethyl, and in which formula the x is an integer less than 3 (that is from 0, l, or 2), the y is from 6 to 10 and the z is either 1 or 2. l

In the accompanying drawing, which is representative of a liow sheet for the process, solvent (eg, dimethyl sulfoxide) is charged from solvent storage tank 1 to the reactor 2 equipped with heating, stirring and condensing apparatus 3, 4 and 5 respectively. A metal carbonate from storage bin 6 and urea from storage bin 7 are x es charged to therreactor,

`Arrinertgas'lsuch as nitrogen .p

from supply S is, preferably, sparged through the reactor to assist in the flow of byproducts consisting mainly of ammonia, carbon-dioxide and water vapor-*through condenser and byproducts dischargeline 9.' Heating and stirring are continued to a temperature 'of about 185 C.

After ammonia evolutionceases, heating, is discontinued. The reaction product in solution is transferred to and separated (e.g'., by centrifuge) and Washed with inert, volatile, non-aqueous liquor (e.g., dichloromethane) utilizing suitable apparatus 11. rThe volatile liquor'is supplied from storage ltank 12,;l The reaction solvent and volatile wash liquor(e.g., dimethyl Vsulfoxide and dichloromethane respectively) in thepltrate are separated by to storage tanks 1 and 12 respectively. The product Vis dried in drier V14 and packaged inweigh anddrum station 15.

The drawing4 can be interpreted in conjunction vwith the description of the preparation of alkali metal and alkaline earth metal cyanatesfin .the following examples:

Example l 26.5 parts by wt., 25 moles of sodium carbonate and 35 parts by wt., 58 rnoles of urea were dissolved in 220 parts by wt., 20 liters, ofdimethyl sulfoxide in a ceramic lined reaction kettle and reacted with gradual heating and stirring A moderate stream of nitrogen gas was passed through n the mass simultaneously. On 'reaching a ,temperature of Y about 160 C. water vapor began'to distill over and at vcooled in crystallizer and the metalcyanate slurry `distillationvin the solvent recovery still 13 and recycled at 188 C. no more ammonia isrelcased.y On cooling, white ilaky'magnesium` Vcyanate'precipitates from the Y solution; it is ltered olf, Washed with parts by weight of pentane, dried and weighed.

magnesium cyanate product is 9.8,partsrby weight, or 91 weight percentof theoretical yield I Y The -Water as well as waterformed inthe reaction from the lreaction system andthe product.' Thus, lower cost carbonates and urea containing water as anrimpurity can be.. used satisfactorily. Y,

VWhile'reaction Vtemperatures in the'range of `100 to 200 C. have been generally preferred, this is an operating condition which can bevaried Awith the solvent used.

Dimethyl sulfoxide `boiling at 189 C.f. is exceptionally 'well suited' to the process for the synthesis of sodium and potassium cyanate. However, other stable polar organic compounds having high solvencyfor metal carbonates and covering a moderately broader boiling range of the order'of 80 to 250 C., such asfdimethyl sulfone, diethyl V"sulfo'xide, and tetramethylene sulfoxide can be used, provided` thaty thev solvent is lstable and unreactive with the .reactants under the conditions of operation, which, in

185 A C. a constant boilingmixture of Water and `dimethyl 'L sulfoxide distilled over. Within a few minutes of Vreach- Ving 185 C. ammonia evolution ceased; heating was'terminated, the distillationwas stopped and the mixture` cooled.r On cooling to ambient temperature',the sodium Y cyanate separatedA as a'white' crystalline solid, which on filtering, Washing Vwith one liter of dichloromethane and Y drying amounted-to 31 parts by weight of sodiumy cyanate v (equivalent to 48 moles), or 9,5% Vof the theoretical yield. i `The Yproduct was'carbonate-free .by barium'nitrate tests,

giving at the most a slight opalescence from precipitated barium carbonate. HByzthe cobalt nitrate test the'product gave the deep blue color of the cyanate complex. y

n Example vlll Y In a manner similar to Example I above, Y n 690 parts'V by weight, 5.0 `moles' of ground anhydrous potassium carbonate and v 780 parts byweight, 13.0 moles of urea dissolved'in 4400 parts Vby weight, liters, of methyl ethyl sulfoxide any event, will be substantially below therSOO to 600 C. Vtemperature offusionof the expected alkalimetal or alkaline earthmetal cyanates, or a reaction product consisting principallyy of 'the' kexpected metal-cyanates.

The examples, as described above made use of gaseous Ynitrogen or lC02 sparged through the reaction mass to vfacilitate the removal of. water from the system. While such operation is benecialin maintaining an anhydrous conditioninthe reactor and ythus'assuring high yields of practically carbonate-free metal cyanate, it is not critical for `the success of `the process.k Thei reaction itself releases two molesY of ammonia,;one., mole of carbon dioxide and 'one mole of water' (vapor) per mole of metal carbonate reacting, so that aV regular `effervescence is Y 'noted/'as the synthesis proceeds. vAlso, other gasesY un- .Y reactive under operating conditions can be usedvr in the. place of'nitrogen and CO2, such as additional gaseous amlmomia', tiue gas (CO2 and N2), hydrogen, helium lor V5o 'f Y The liquid reaction mass is heated and stirredvvhile" nitrogen gasis passed gently therethrough. Within onel hour the temperature Vis brought up to 180 C. withwaterVV vapor coming off alongrwith both ammonia andl CO2'.

. After about 1/ihour andata temperature of 195 C. a sample taken from the reaction product gives a negligible j test for carbonate by the4 barium nitrate test. Heatingjis terminated at,- this point and the mass cooled to room temperature. Crude crystallized potassiumv cyanate (ap-AVV proximately l750v parts by weight, 94%; of ktheoretical yield) is separated by filtration; After slurrying and Vwashing, the crude crystals with three successive 0.2 liter Vamounts of absolute alcohol, the potassium cyanate prod- Y l uct( is 7140 parts by weight, or %Y of theoretical yield.

n 'Example `1117 8.4 parts vby weight of magnesium carbonate 12.5 Vparts by weight of urea arel dissolved slowly in 210 parts byweight of dimethyl sulfonefwith heating andY l Alkali metal Vand alkaline earth metal cyanates, as'the saltsol:` cyanic acid, have vfound many uses in industry 'and particularly'in organicsynthesis. For example, potassiumrcyanate and dialkyl sulfatesmay Vbe reacted to ",2KoCN+R2so,+ '2RNCo+K2so, n ,whichV can provide' selected alkyl isocyanates forurethane synthesisi Y stirring. At atemperature above C., water vapor, Y* i CO2 and ammonia are released as the reaction Vmass effervesces. p ternal source -is passed through the reactionmass until,

A streamv of gaseous CO2'from an ex- 1 VThe process of the instant invention is, furthermore, of specilic interest in that the metal cyanates -formed in the presence of an excess ofdimeth'yl sulfoxide,.for example,

KVVthe total reaction productof the above process, can. be Aused directly in thes'ynthesis ofselected alkyl or alkenyl a substituted urethanes. With warming, the metal cyanate in dirnethylsulfoxide solution isfreacted with an alkyl or alkenyl halideV and an alcoholjto form alkyl alkenyl urethanes in highpurity an excellent yield. Moreover,.the

derived,V alkyll alkenyl `urethane vreaction product, which is readily "freed' of insoluble metal.salts,rsti1l inthepresence of a major proportion of dimethyl sulfoxide, can be copolymerized directly with polymerizable chain extending monomers, such as vinyl acetate, and polymerizable plasticizer-type compounds, such as dibutyl maleate, in the vpresence of polymerization catalysts, suchy as azoaies/,asv

y as foam stabilizers in foamed polyurethanes.

Lithium, potassium and related cyanates are used kextensively in glass making and the ceramic arts as well as in producing protective coatings on metals. Metal and organic' cyanates have, furthermore, been used extensively as herbicides in weed control, as fertilizers, insecticides and as agents to control methane conversion of distillery Wastes.

Obviously many modications and variations ofthe invention as hereinabove set forth may be made without departing from the spirit and scope of the invention, and therefore, only such limitations should be imposed as are indicated in the appended claims.

What is claimed is:

1. `The method of synthesizing a metal cyanate selected from the group consisting of lithium, sodium, potassium,

rubidium, caesium, beryllium, magnesium, calcium and strontium cyanate from the corresponding' metal carbonates and urea which comprises: y

(a) 'reacting at a temperature in the range from about 80 C. to about 250 C. a solution of the metal carbonate and urea dissolved in a solvent selected from the group consisting of dimethyl sulfoxide, methyl ethyl sulfoxide, and dimethyl sulione;

(b) removing volatile byproducts formed during the approximately fifteen minutes of such reaction; and

(c) cooling, crystallizing and recovering the resulting metal cyanate.

' 2. The method as set forth lin claim 1, wherein the y' heated reactants are purged of water vapor and volatile 3. The method as set forth in claim 1, wherein the recovered metal cyanate is washed with a volatilizable, nonreactive liquid of the group consisting of hydrocarbons, halogenated hydrocarbons and alcohols.

4. The method of synthesizing sodium cyanate from sodium carbonate and urea which comprises heating and stirring a dimethyl sulfoxide solution of sodium carbonate and urea at a temperture of about C. until ammonia'evolution ceases; thereafter cooling the solution to recover sodium cyanate; and washing the sodium cyanate with a volatile liquid of the group consisting of pentane, dichloromethane, and absolute alcohol.

References Cited in the le of this patent UNITED STATES PATENTS 1,915,425 Kloepfer June 27, 1933 2,699,418 Ter Horst Jan. 1l, 1955 2,729,541 De Pree et al. Ian. 3, 1956 2,889,198 Barrett et al June 2, 1959 FOREIGN PATENTS 354,604 Great Britain Aug. 13, 1931 OTHER REFERENCES The Condensed Chemical Dictionary, 6th Edition, page 399, Reinhold Pub. Corp., New York, 1961. Q.D.5, C5, 1961, C. 17.

Potassium Cyanate article in Cyanamid New Product Bulletin, Collective Volume I (Revised Edition), published by American Cyanamid Co., 30 Rockefeller Plaza, New York 20, N.Y., October 1952, page 130. 

1. THE METHOD OF SYNTHESIZING A METAL CYANATE SELECTED FROM THE GROUP CONSISTING OF LITHIUM, SODIUM, POTASSIUM, RUBIDIUM, CAESIUM, BERYLLIUM, MAGNESIUM, CALCIUM AND STRONTIUM CYANATE FROM THE CORRESPONDING METAL CARBONATES AND UREA WHICH COMPRISES: (A) REACTING AT A TEMPERATURE IN THE RANGE FROM ABOUT 80*C. TO ABOUT 250*C. A SOLUTION OF THE METAL CARBONATE AND UREA DISSOLVED IN A SOLVENT SELECTED FROM THE GROUP CONSISTING OF DIMETHYL SULFOXIDE, METHYL ETHYL, SULFOXIDE, AND DIMETHYL SULFONE: (B) REMOVING VOLATILE BYPRODUCTS FORMED DURING THE APROXIMATELY FIFTEEN MINUTES OF SUCH REACTION; AND (C) COOLING, CRYSTALLIZING AND RECOVERING THE RESULTING METAL CYANATE. 